A centrifugal compressor is required to have high pressure and high efficiency in a wide operation range.
FIG. 5 is an enlarged cross-sectional view of a principal portion of the upper half of the centrifugal compressor above the rotational axis center of a compressor impeller.
A compressor 1 of the centrifugal compressor includes a turbine rotor 2 in which a compressor impeller 3 mainly comprised of a rotary hub 31 and a large number of centrifugal blades 32 attached to the outer peripheral surface of the hub 31 is coupled to a rotation drive source using a shaft, and a compressor housing 11 which accommodates the turbine rotor 2 and forms a flow path of a fluid.
In the compressor housing 11, there are provided a diffuser portion 13 which is formed on the outer peripheral side of the compressor impeller 3 in a substantially toroidal shape and recovers a static pressure by decelerating an air flow discharged from the compressor impeller 3, and a scroll 12 which is formed on the outer peripheral side of the diffuser portion 13 such that the cross-sectional area thereof is spirally increased in a circumferential direction, and decelerates and pressurizes the air flow, and an exit pipe (not shown).
When the compressor impeller 3 rotates, the centrifugal blades 32 compress a fluid such as gas or air introduced from an air passage 15. The flow of the fluid (air flow) formed in this manner passes through the diffuser portion 13 and the scroll 12 from the outer peripheral end of the compressor impeller 3 to be sent to the outside from the exit pipe.
FIG. 6 is a schematic view of the scroll 12 as viewed in plan view.
In FIG. 6, the distribution of a radius R at each of positions set at intervals of 30° from the position at 60° in a clockwise direction with a scroll end point (360° in FIG. 6) used as a 0 reference (0° on a circular coordinate) is constant.
In FIG. 7(A), the horizontal axis indicates a circumferential angle position, while the vertical axis indicates a radius R from a rotational axis center L1 of the compressor of the scroll 12 to a scroll centroid P. FIG. 7(A) shows that the distribution of the radius R is constant.
In addition, FIG. 7(B) is a view in which cross sections of the scroll 12 at individual circumferential positions (set at intervals of 30°) in the clockwise direction relative to the position at 60° in FIG. 6 are stacked and shown, and shows the change of the scroll centroid P in the direction of the radius R.
As a conventional art in which the structure of the scroll is changed, Japanese Patent Application Laid-open No. 2010-209824 (Patent Document 1) is disclosed.
In Patent Document 1, the outer edge of a scroll portion having a flow path spirally formed around the rotational axis of a rotor blade of a turbine which obtains power by supplying fluid gas to the rotor blade includes an arc portion which is formed such that a radius R is constant and a portion in which the radius R is gradually decreased from the end of the arc portion to the end of the scroll portion.
Patent Document 1: Japanese Patent Application Laid-open No. 2010-209824
However, in the technology of Patent Document 1, the scroll structure is for the turbine which obtains power while supplying the fluid gas to the rotor blade to cause expansion, and is different from the scroll structure of the present application which compresses the air flow in the way of flow and nature of the air flow.
In addition, the centrifugal compressor obtains the static pressure by decelerating the flow accelerated by the compressor impeller 3.
In the conventional art as shown in FIG. 7, the deceleration of the air flow is carried out by the diffuser portion 13, and the air flow is not decelerated in the scroll, which results in a loss in the scroll 12 so that it has not been possible to obtain high efficiency and high pressure as the centrifugal compressor.
On the other hand, when the air flow is decelerated in the scroll 12, the thickness of a boundary layer generated between the scroll wall and the fluid is increased so that the static pressure cannot be adequately recovered.